1. Field of the Invention
The present invention relates generally to methods for chemical vapor deposition (CVD) of silicon nitride, and more particularly to a method for CVD of silicon nitride employing a novel combination of flow rate, temperature and pressure to achieve improved film properties at a high rate of deposition at low pressure.
2. Brief Description of the Prior Art
Silicon nitride is commonly used in the manufacturing of semiconductor devices wherein it is deposited onto substrates (i.e. wafers) by chemical vapor deposition. Deposition is carried out in a variety of commercially available hot wall and cold wall reactors by placing a substrate in a vacuum chamber, heating the substrate and introducing dichlorosilane and ammonia. Deposition rates of approximately 10-70 angstroms per minute are achieved for low pressure processes (less than 1 Torr) as described in Chemical Vapor Deposition for Microelectronics (A. Sherman, Noyes Publications, Park Ridge, N.J. (1987), p. 77), and 50-100 angstroms per minute for high pressure processes (10-200 Torr) as described in detail in U.S. Pat. Nos. 5,482,739 and 5,629,043. Higher deposition rates of 2,000 angstroms per minute have been reported for plasma enhanced depositions as reported in U.S. Pat. No. 5,399,387, however silicon nitride films deposited by plasma enhancement do not have the same properties as silicon nitride deposited without plasma enhancement.
A typical prior art vertical furnace low pressure chemical vapor deposition (LPCVD) system is depicted in FIG. 1 and includes a chamber having a quartz tube 10 and chamber seal plate 12 into which is inserted a boat 14 for carrying a plurality of substrates 16. Dichlorosilane and ammonia gases enter the gas injection tube (or tubes) 18 from the gas inlet tube (or tubes) 20 through the chamber seal plate 12. The gases exit the process chamber through the seal plate 12 and out the exhaust port 24. A plurality of heater elements 26 are separately controlled and adjustable to compensate for the well-known depletion of the feed gas concentration as the gas flows from the gas injection tube 18 to the chamber exhaust port 24. This type of deposition system typically operates in the 200 mTorr to 500 mTorr range (200xc3x9710xe2x88x923 Torr to 500xc3x9710xe2x88x923 Torr). Typical gas flows are 30 sccm of dichlorosilane, 200 sccm of ammonia and 1-2 slm of a carrier gas such as nitrogen or hydrogen. Operating at these low concentrations of dichlorosilane and ammonia results in low deposition rates of typically 10-60 angstroms per minute. Operation at higher concentrations of reactant gases results in non-uniform deposition across the substrates and great differences in the deposition rate from substrate to substrate. Increased flow rates could improve the deposition uniformity at higher pressures, however increased gas flow increases the reactive gas pressure at the injection tube holes causing gas phase nucleation resulting in particles being deposited on the substrates. Other problems associated with this reactor include film deposition on the interior quartz tube 10 and gas injection tube 18. This unwanted deposition decreases the partial pressure of the reactive feed gas concentration near the surface of the substrate 16 resulting in a reduced deposition rate and potential contamination caused when film deposited on the wall of tube 10 and injector tube 18 flakes off and deposits on the substrates 16. In addition, a major problem is the accumulation of reactant gas by-products, mainly composed of ammonium chlorides at the exhaust tube resulting in contamination of the substrates. Attempts have been made to deposit silicon nitride using silane in place of dichlorosilane to eliminate the ammonium chloride by products, however such attempts have resulted in non-acceptable film uniformity.
Another prior art reactor is illustrated in FIG. 2 and described in detail in U.S. Pat. No. 5,108,792. A substrate 28 is placed on a rotating substrate carrier 30, enclosed in a vacuum tight chamber having an upper quartz dome 32 and a lower quartz dome 34 and associated chamber wall 36. The substrate 28 is heated by upper lamps 38 and lower lamps 40. Reactant gases are injected through gas input tube 42 and exhausted through exhaust tube 44. This reactor overcomes some of the limitations of the vertical furnace reactor of FIG. 1. The reactor can be operated at higher pressures than vertical LPCVD furnaces and does not have an injector tube and its associated problems.
U.S. Pat. No. 5,482,739 entitled xe2x80x9cSilicon Nitride Depositionxe2x80x9d describes the process conditions and reactor modifications required to deposit silicon nitride at a rate of 70 angstroms per minute in the reactor described in U.S. Pat. No. 5,108,792. The reactor modifications reduce the accumulation of ammonium chloride by-products. The typical process pressure range is 10-200 Torr for depositing silicon nitride on a silicon substrate. This process pressure is approximately 2 orders of magnitude higher than the nominal deposition pressure of a similar LPCVD process.
Increased deposition rates are very desirable, resulting in higher machine productivity and more importantly reducing the time the substrates are exposed to high temperatures, i.e.  greater than 600xc2x0 C. Reduced time at high temperatures is important during the fabrication of semiconductor devices as the device sizes become smaller, because elevated temperatures, i.e.  greater than 600xc2x0 C., for any extended time cause unwanted changes in semiconductor device structure. A disadvantage of the prior art low pressure methods is their low deposition rate. A disadvantage of the prior art high pressure methods is that operating at high pressure can cause a gas phase reaction which can produce particulate contamination on the wafer.
U.S. Pat. No. 5,551,985 by Brors et al. describes a CVD reactor that provides improved uniformity in heating a wafer, and a highly uniform gas flow across the surface of a wafer. U.S. patent application Ser. No. 08/909,461 filed on Aug. 11, 1997, and Ser. Nos. 09/228,835 and 09/228,840 filed on Jan. 12, 1999, the disclosures of which are incorporated herein by reference, describe wafer chambers in which related processes may also be used.
It is an object of the present invention to provide a method for rapid deposition of a silicon nitride film having a highly uniform, smooth surface.
It is a further object of the present invention to provide a method of operating a CVD reactor that optimizes the rate and uniformity of deposition of silicon nitride.
It is a still further object of the present invention to provide a method of operating a CVD reactor that results in a high degree of uniformity in deposition of silicon nitride from one run to another.
It is another object of the present invention to provide a method of operating a CVD reactor that optimizes the rate and uniformity of deposition of silicon nitride, and substantially reduces the accumulation of ammonium chloride by-products.
Briefly, a preferred embodiment of the present invention includes a method of operating a CVD reactor providing a novel combination of wafer temperature, gas flow and chamber pressure resulting in both rapid deposition and a uniform, smooth film surface. According to the method, a wafer is placed in a vacuum chamber wherein a reactant gas flow of silane and ammonia is directed in parallel with the wafer surface via a plurality of temperature controlled gas injectors, the gas being confined to a narrow region above the wafer. The gas is injected at a high velocity, causing the deposition rate to be limited only by the rate of delivery of unreacted gas to the wafer surface and the rate of removal of reaction byproducts. The high velocity gas stream passing across the wafer has the effect of thinning the layer adjacent the wafer surface containing reaction by-products, known as the xe2x80x9cboundary layer,xe2x80x9d resulting in faster delivery of the desired reactant gas to the wafer surface. The rapid gas flow also reduces the gas residence time above the substrate surface, and sweeps out unwanted reaction by-products resulting in a further increase in the relative concentration of the desired reactant species and reduced incorporation of unwanted reaction by-products in the deposited film.
An advantage of the present invention is that it results in more rapid deposition of silicon nitride.
A further advantage of the present invention is that it provides a process resulting in a reduced incorporation of unwanted by-products in a deposited silicon nitride film.
A still further advantage of the present invention is that it provides a method resulting in higher rate deposition of silicon nitride with improved film uniformity and reproducibility between batches.